The emission of air toxics from combustion sources is an important issue in light of the 1990 Clean Air Act Amendments. Title III of these amendments governs air toxics and, in general, requires major emission sources to control the amount of air toxics emissions to less than 10 tons per year for any one toxic species and less than 25 tons per year for all species. Air toxics are present in the flue gases of combustion sources and appear both as particulate metals, such as nickel, arsenic and chromium, in the fly ash particles and vapor phase metals, such as mercury, selenium and halides and organic vapors.
Trace metals in flue gas are normally concentrated in the fly ash particles and can be removed effectively by an efficient particulate collector such as an electrostatic precipitator or a baghouse. Methods have been provided for removing particulate phase air toxics from the flue gas of a combustion source. For example, Smith et al. (U.S. Pat. No. 4,956,162) discloses a method for removing particulate matter, sulfur dioxide and volatile toxic materials from the flue gas of coal-fired boilers. In general, the process of Smith et al. includes directing the flue gas to an electrostatic precipitator (ESP) for removal of particulate ash and dust from the flue gas. Dry sodium based or calcium based sorbents are blown continuously into the relatively particle-free flue gas between the ESP and a baghouse to react with the sulfur dioxide. The sorbents/sulfur dioxide byproduct and unreacted sorbent are collected on the filter bags of the baghouse where reaction and collection of sulfur dioxide continues to occur. The material collected on the filter bags is removed periodically in a normal fashion.
Another method for filtering fly ash and other particulates from flue gas is described in Chang (U.S. Pat. No. 5,024,681) and includes the steps of passing the gas through an ESP to remove most of the particulates from the flue gas then through a baghouse to remove the remaining particulates. The particulates exiting the ESP are at relatively low concentrations and are charged so as to have lower penetration across the bag filter than uncharged particles. These charged particles also tend to build a dustcake with low flow resistance. The combination of low dust loading, low dustcake flow resistance and low particle penetration allow the relatively compact baghouse downstream of the ESP to operate at high face velocities.
Vapor phase air toxics are present in flue gas in very small concentrations, for example parts per billion to parts per trillion quantities, which are difficult to remove. To date, specially designed and costly emissions-control systems have been required to capture these volatile compounds effectively.
Some of the techniques currently used for removing mercury from gas streams include either injecting sorbents into the gas stream before a particulate collection device, passing the gas stream through a packed bed of sorbent or using a wet system to capture mercury compounds. For example, Smith et al. discussed above provides that if the combustion gas is cooled before entering the baghouse, volatile toxic materials, which are in vapor phase at flue gas temperatures but condense on the fly ash particles at lower temperatures, are removed as well. Injected sorbents are most commonly activated carbons or carbons treated chemically to produce sulfide or iodide compounds with mercury. Packed-bed adsorbers typically use carbon-based materials for collecting mercury. Wet systems involve equipment such as scrubber towers, spray dryers, wet electrostatic precipitators or water-treatment facilities. Work on the removal of mercury from flue-gas streams has involved the use of injected adsorbents and wet chemicals.
In general, these currently provided systems for removing vapor phase air toxics from flue gas require relatively large quantities of sorbent and produce relatively large quantities of hazardous waste. Furthermore, the prior art practice of injecting sorbent continuously into the gas stream to react with air toxics therein has not proven effective since reactions between the sorbent and trace air toxics in the gas stream are limited by mass transfer and kinetic reaction.